Differential spectral liquid level sensor

ABSTRACT

Systems and methods that use a differential spectral liquid level sensor to measure the level of liquid in a reservoir (e.g., a fuel tank or other storage container). The use of a differential spectral liquid level sensor solves the problem of common-mode intensity variations (i.e., intensity variations not due to the level of the liquid) by having two different wavelengths propagate through the same optical path but have different spectral attenuations in the liquid. By determining the ratio of the received optical powers, common-mode intensity variations can be neutralized, thereby enhancing the accuracy of the received power reading and the resulting liquid level indication.

RELATED PATENT APPLICATION

This application is a divisional of and claims priority from U.S. patentapplication Ser. No. 15/488,932 filed on Apr. 17, 2017, which issued asU.S. Pat. No. 10,371,559 on Aug. 6, 2019.

BACKGROUND

This disclosure generally relates to systems and methods for measuring alevel of liquid in a reservoir, such as a storage tank or othercontainer. More particularly, this disclosure relates to systems andmethods for liquid level measurement using an optical sensor.

The level of a liquid is continuously measured in many commercial andmilitary applications. For example, liquid-level sensors are commonlyused in the fuel tanks of aircraft, automobiles, and trucks.Liquid-level sensors are also used to monitor liquid levels withinstorage tanks used for fuel dispensing, wastewater treatment, chemicalstorage, food processing, etc.

Many transducers for measuring liquid level employ electricity. Theelectrical output of such transducers changes in response to a change inthe liquid level being measured, and is typically in the form of achange in resistance, capacitance, current flow, magnetic field,frequency, and so on. These types of transducers may include variablecapacitors or resistors, optical components, Hall Effect sensors, straingauges, ultrasonic devices, and so on.

Currently most fuel sensors on aircraft use electricity. For example,existing electrical capacitance sensors require electrical wiring insidethe tank, which in turn requires complex installations and protectionmeasures to preclude a safety issue under certain electrical faultconditions. This electrical wiring requires careful shielding, bonding,and grounding to minimize stray capacitance and further requiresperiodic maintenance to ensure electrical contact integrity.

A simplex (non-differential) optical impedance fuel level sensor basedon optical intensity measurement has been proposed which would eliminateall electrical elements. One such optical impedance fuel level sensorcomprises two optical fibers spaced apart inside a meniscus tube: aside-emitting optical fiber that transmits light along its length and aside-receiving optical fiber that receives emitted light along itslength. The meniscus tube minimizes the sloshing of fuel level. Thevariable fuel level in the tank produces changes in the opticalimpedance between the two optical fibers, resulting in changes in thetotal light received by an optical detector.

However, the aforementioned simplex optical impedance fuel level sensoris susceptible to inaccuracy due to intensity variations along theoptical path that are not related to fuel level. These intensityvariations may be attributable to one or more of the following factors:(1) temperature variation; (2) surface tension wetting (non-shedding ofliquid); (3) fuel gunk buildup on the optical window surface of thefiber sensor elements; (4) ice slush in the lower portion of the fueltank due to water condensation and cold temperature; (5) fuel surfacetilt in a dynamic flight environment; (6) fiber attenuation due toaging; (7) fiber attenuation due to bending; (8) connector attenuationdue to alignment; (9) non-uniformity of light emitting along the lengthof the side-emitting optical fiber due to manufacturing imperfection;and (10) non-uniformity of light received along the length of theside-receiving optical fiber due to manufacturing imperfection.

It would be desirable to provide a liquid level sensor that measures theoptical impedance of light propagating through the liquid in a mannerthat is not corrupted by one or more of the aforementioned sources ofintensity variation.

SUMMARY

The subject matter disclosed herein is directed to improvements insystems and methods that use an optical impedance sensor to measure thelevel of liquid in a reservoir (e.g., a fuel tank or other storagecontainer). More specifically, various embodiments of a differentialspectral liquid level sensor disclosed below solve the problem ofcommon-mode intensity variations by having two different wavelengthspropagate through the same optical path but have different spectralattenuations in the liquid. By determining the ratio of the receivedoptical powers, common-mode intensity variations can be neutralized,thereby enhancing the accuracy of the received power reading and theresulting liquid level indication.

In accordance with some embodiments disclosed in some detail below, adifferential spectral fuel level sensor is provided that employs lightof two different wavelengths that both travel the same optical paththrough a column of fuel in a fuel tank. The light of two wavelengths isoptically coupled into a side-emitting optical fiber disposed verticallyin the fuel tank. The side-emitting optical fiber emits some of thereceived light along its length toward a side-receiving optical fiber,also disposed vertically inside the fuel tank. The side-emitting andside-receiving optical fibers are spaced apart inside a meniscus tube.The meniscus tube minimizes the sloshing of fuel level. The variablefuel level in the fuel tank produces changes in the optical impedancebetween the two optical fibers, resulting in changes in the total lightreceived by any optical detector optically coupled to the side-emittingoptical fiber. Some of the light received along the length of theside-receiving optical fiber exits the upper end of the side-receivingoptical fiber and is optically coupled to a differential receiver. Inaccordance with one embodiment, the differential receiver comprises apair of optical filters and an associated pair of optical detectors. Inaccordance with another embodiment, the differential receiver comprisesa dichroic mirror and a pair of optical detectors. For the samedifferential spectral sensing functionality, one broadband opticalsource can be used in place of two different optical sources, or oneoptical detector can be used in place of a differential receiver bytime-division multiplexing of the two different wavelength sources. (Theterms “optical source” and “light source”, as used in this disclosure,are synonymous.) Since the fuel level measurement is based on opticalintensity, many other intensity variations that are not related to fuellevel can affect the received optical power and therefore create errorin fuel level measurement. These other (i.e., common-mode) intensityvariations may include temperature, component aging, fuel gunk depositon the sensor elements, etc. By using differential wavelengths, theratio of the received optical powers can be calculated, which rationeutralizes intensity variations that are not due to fuel level.

Although various embodiments of systems and methods for opticallymeasuring the level of fuel in a fuel tank will be described in somedetail below, one or more of those embodiments may be characterized byone or more of the following aspects.

One aspect of the subject matter disclosed in detail below is a systemfor measuring a level of liquid in a reservoir, comprising: one opticalsource which alone or two optical sources which collectively outputslight comprising a first wavelength and light comprising a secondwavelength different than the first wavelength; a side-emitting opticalfiber optically coupled to the one or two optical source(s); aside-receiving optical fiber that is positioned parallel to and at adistance from the side-emitting optical fiber; and a differentialreceiver optically coupled to the side-receiving optical fiber andconfigured to convert light having the first wavelength into firstelectrical signals and convert light having the second wavelength intosecond electrical signals. The light comprising a first wavelength andthe light comprising a second wavelength have different attenuationswhen propagating through the liquid.

In accordance with some embodiments, the system further comprises acomputer system configured to calculate an estimated level of liquid inthe reservoir based on a difference of the first and second electricalsignals output by the differential receiver. In addition, the system mayfurther comprise a display device electrically coupled to the computingsystem, in which case the computing system is further configured toexecute the following operations: storing data representing a geometryof the reservoir; receiving data representing a measurement of a densityof the liquid in the reservoir; calculating a mass of liquid remainingin the reservoir based on the geometry of the reservoir, the density ofthe liquid and the estimated level of liquid; and outputting anelectrical signal representing the calculated mass of liquid in thereservoir to the display device.

In accordance with one embodiment, the differential receiver comprises:a first optical filter that passes light having the first wavelength,but does not pass light having the second wavelength; a first opticaldetector that receives light that has passed through the first opticalfilter; a second optical filter that passes light having the secondwavelength, but does not pass light having the first wavelength; and asecond optical detector that receives light that has passed through thesecond optical filter. In accordance with another embodiment, thedifferential receiver comprises: a dichroic mirror that passes lighthaving the first wavelength and reflects light having the secondwavelength; a first optical detector positioned to receive light passedby the dichroic mirror; and a second optical detector positioned toreceive light reflected by the dichroic mirror.

Another aspect of the subject matter disclosed in detail below is asystem for measuring a level of liquid in a reservoir, comprising: afirst optical source that outputs light comprising a first wavelength; asecond optical source that outputs light comprising a second wavelengthdifferent than the first wavelength; a time-division multiplexingcontroller configured to control the first and second optical sources tooutput time-division multiplexed optical pulses having the first andsecond wavelengths in alternating sequence; a side-emitting opticalfiber that is optically coupled to the first and second optical sources;a side-receiving optical fiber that is positioned parallel to and at adistance from the side-emitting optical fiber; an optical detector thatis optically coupled to receive light from the side-receiving opticalfiber and convert the received light to electrical signals; and atime-division demultiplexer that is electrically coupled to the opticaldetector and comprises switches which are controlled to demultiplex theelectrical signals output by the optical detector. In accordance withsome embodiments, the system further comprises a computer systemconfigured to calculate an estimated level of liquid in the reservoirbased on a difference of the demultiplexed signals output from thetime-division demultiplexer.

A further aspect of the subject matter disclosed in detail below is amethod for measuring a height of liquid in a reservoir, comprising:placing a side-emitting optical fiber and a side-receiving optical fiberin the reservoir having respective locations whereat the side-emittingoptical fiber and side-receiving optical fiber are mutually parallel andseparated by a distance; outputting light comprising a first wavelengthand light comprising a second wavelength different than the firstwavelength from a single broadband optical source or from respectiveoptical sources, wherein the light comprising a first wavelength and thelight comprising a second wavelength have different attenuations whenpropagating through the liquid; guiding the light comprising the firstand second wavelengths into the side-emitting optical fiber;side-emitting at least some of the light received by the side-emittingoptical fiber toward the side-receiving optical fiber; guiding at leastsome of the light comprising the first wavelength received by theside-receiving optical fiber onto a first optical detector; guiding atleast some of the light comprising the second wavelength received by theside-receiving optical fiber onto a second optical detector; convertinglight having the first wavelength that impinges on the first opticaldetector into first electrical signals; converting light having thesecond wavelength that impinges on the second optical detector intosecond electrical signals; calculating an estimated level of liquid inthe reservoir based on a difference of the first and second electricalsignals; and displaying a fuel gauge that indicates the estimated levelof liquid.

Yet another aspect is a method for measuring a height of liquid in areservoir, comprising: placing a side-emitting optical fiber and aside-receiving optical fiber in the reservoir having respectivelocations whereat the side-emitting optical fiber and side-receivingoptical fiber are mutually parallel and separated by a distance; guidinga series of time-division-multiplexed optical pulses into one end of theside-emitting optical fiber, wherein the time-division-multiplexedoptical pulses comprise alternating optical pulses having the first andsecond wavelengths respectively; side-emitting at least some of theoptical pulses received by the side-emitting optical fiber toward theside-receiving optical fiber; guiding at least some of thetime-division-multiplexed optical pulses received and output by theside-receiving optical fiber onto an optical detector; convertingtime-division-multiplexed optical pulses that impinge on the opticaldetector into time-division-multiplexed electrical signals;demultiplexing the time-division-multiplexed electrical signals outputby the optical detector to generate first and second electrical signals;and calculating an estimated level of liquid in the reservoir based on adifference of the first and second electrical signals.

In accordance with some embodiments, each of the methods brieflydescribed in the preceding two paragraphs further comprises: storingdata representing a geometry of the reservoir; measuring a density ofthe liquid in the reservoir; calculating a mass of liquid remaining inthe reservoir based on the geometry of the reservoir, the density of theliquid and the estimated level of liquid; and displaying a gauge thatindicates the calculated mass of liquid in the reservoir.

Other aspects of differential spectral liquid level sensors suitable foruse in reservoirs are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, functions and advantages discussed in the precedingsection can be achieved independently in various embodiments or may becombined in yet other embodiments. Various embodiments will behereinafter described with reference to drawings for the purpose ofillustrating the above-described and other aspects.

FIG. 1 is a hybrid diagram representing a system for measuring a levelof a liquid comprising an optical impedance sensor that detectsmodulations in the optical impedance of the liquid. This hybrid diagramcomprises a block diagram showing components of an optical transceiverand a diagram representing an isometric view of the optical impedancesensor. The nonlinear line spanning the sensor represents a level ofliquid; the arrows represent photons propagating from one optical fiberto another optical fiber.

FIG. 2 is a diagram representing a side-emitting optical fiber and aside-receiving optical fiber separated by a distance d inside a fueltank.

FIG. 3 is a hybrid diagram representing some components of adifferential spectral liquid level sensor comprising a pair ofdifferential optical sources and a pair of differential opticaldetectors in accordance with a first embodiment.

FIG. 4 is a diagram representing a top view of an optical Y-coupler inaccordance with one embodiment, which optical Y-coupler may be used aseither a combiner or a splitter depending on the direction of the lightpropagating therethrough.

FIG. 5 is a hybrid diagram representing some components of adifferential spectral liquid level sensor comprising a broadband opticalsource and a pair of differential optical detectors in accordance with asecond embodiment.

FIG. 6 is a hybrid diagram representing some components of adifferential spectral liquid level sensor comprising a pair ofdifferential optical sources and an optical detector in accordance witha third embodiment.

FIG. 7 is a graph showing two optical pulses in accordance with atime-division-multiplexing scheme used in the third embodiment partlydepicted in FIG. 6. The vertical axis measures optical pulse amplitude;the horizontal axis measures time.

FIG. 8 is a diagram representing a top view of a dichroic mirror whichcan be used as a beam splitter in place of the optical Y-couplerdepicted in FIG. 4.

FIG. 9 is a diagram representing an elevation view of an optical fiberhaving a mirror cap at one end for increasing the intensity of lightinside the fiber.

FIG. 10 is a diagram representing a plan view of a pair of opticalfibers encased in respective jackets having mutually opposinglongitudinal slots for sideways optical coupling of light (indicated byarrows) from the side-emitting optical fiber to the side-receivingoptical fiber.

FIG. 11 is a diagram representing a plan view of a pair of opticalfibers encased in respective jackets having mutually opposinglongitudinal slots covered by respective lenses for sideways opticalcoupling of light (indicated by arrows) from the side-emitting opticalfiber to the side-receiving optical fiber.

FIG. 12 is a block diagram identifying some components of a system forproviding a pilot with an indication of the estimated quantity of fuelremaining in the fuel tanks.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

Various embodiments of systems and methods for optical measurement of alevel of liquid in a reservoir will now be described in detail for thepurpose of illustration. At least some of the details disclosed belowrelate to optional features or aspects, which in some applications maybe omitted without departing from the scope of the claims appendedhereto. The disclosed optical impedance fuel level sensor hasapplication in the measurement of the liquid level in a fuel tank of avehicle (such as an airplane) or in other types of liquid storagecontainers, including standing structures. Fuel tanks and other liquidstorage containers are collectively referred to herein as “reservoirs”.

In particular, illustrative embodiments of an optical impedance fuellevel sensor on an airplane are described in some detail below. However,not all features of an actual implementation are described in thisspecification. A person skilled in the art will appreciate that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

FIG. 1 is a hybrid diagram representing a system for measuring a levelof fuel in a reservoir in accordance with the optical impedancemodulation concept. The system depicted in FIG. 1 comprises an opticalimpedance sensor 2 that detects the optical impedance of the fluidseparating a side-emitting optical fiber 4 and a side-receiving opticalfiber 6, obtaining optical power data that can be later used todetermine the fuel level. In accordance with the embodiment shown inFIG. 2, the side-emitting optical fiber 4 is optically coupled to anoptical source 10 (e.g., a laser or a light-emitting diode (LED)) bymeans of an optical fiber 12; and the side-receiving optical fiber 6 isoptically coupled to an optical detector 14 (e.g., a photodiode) bymeans of an optical fiber 16.

The optical impedance sensor 2 further comprises a meniscus tube 8 thatminimizes fuel sloshing in a fuel tank (not shown). The nonlinear linespanning the optical impedance sensor 2 in FIG. 1 represents a level offuel. The side-emitting and side-receiving optical fibers 4 and 6 areplaced inside the meniscus tube 8 in spaced-apart relationship(preferably the fibers are straight and parallel to each other). Incases where the fuel tank is incorporated in a wing of an airplane, theside-emitting and side-receiving optical fibers 4 and 6 are preferablyrigidly supported in a fixed spatial relationship to provide aseparation distance d which is optimized for optical received powerversus ice slush particles that may form in the fuel tank. The meniscustube 8, which extends to the floor of the fuel tank, has openings nearthat floor which allow fuel to flow into the volume of space bounded bythe meniscus tube 8. The level of the fuel will be the same inside andoutside the meniscus tube 8.

When pumped by the optical source 10, the side-emitting optical fiber 4emits light radially outward and toward the side-receiving optical fiber6. The axial distribution of emitted light may be substantially constantalong the length of side-emitting optical fiber 4. A first portion ofthe light will pass through the fuel and illuminates a lower portion ofthe side-receiving optical fiber 6. A second portion of light emitted byside-emitting optical fiber 4 will pass through the air and illuminatean upper portion of the side-receiving optical fiber 6. At least some ofthe light received by side-receiving optical fiber 6 is guided upwardsand other light is guided downwards inside the core of side-receivingoptical fiber 6. The light guided downwards may be reflected upwardsfrom a mirror (not shown in FIG. 1, but see mirror 18 in FIG. 9)disposed at the bottom end of side-receiving optical fiber 6. The lightis guided upwards and exits the upper end of side-receiving opticalfiber 6. The light output by side-receiving optical fiber 6 is guided byoptical fiber 16 to the optical detector 14, which converts impinginglight into electrical current. This electrical current is conducted by acable to a computer or processor (not shown in FIG. 1). The computer orprocessor is configured to analyze the optical power data acquired fromthe optical detector 14 and compute the height h of the air/fuelinterface. As will be explained in more detail below, the computer orprocessor may also be configured to receive fuel temperature data from atemperature sensor and fuel density data from a densitometer and computethe quantity of fuel in the fuel tank based on the optical power, fueldensity data and geometry of the fuel tank (or compartment thereof). Inthis case, the computer may be referred to as a fuel quantity processingunit (FQPU).

The arrows in FIG. 1 represent light (i.e., photons) propagating fromthe side-emitting optical fiber 4 to the side-receiving optical fiber 6during operation of optical source 10. During monitoring of the fuellevel, the brightness (i.e., intensity) of the light produced by opticalsource 10 (i.e., its optical power) is preferably constant. As the fuellevel varies, the optical impedance of the fuel in the volume of spacebetween side-emitting optical fiber 4 and side-receiving optical fiber 6changes in dependence on the fuel level, due to the fact that air andfuel have different refractive indices.

It is well known that air has an index of refraction less than the indexof refraction of fuel; that fuel has an index of refraction less thanthe index of refraction of cladding of an optical fiber; and that thecladding has an index of refraction less than the index of refraction ofthe core of the optical fiber. The refractive indices determine theamount of light that is reflected when reaching an interface.

Since more optical power is lost (i.e., optical impedance is greater) inliquids than in air, the optical power output by the side-receivingoptical fiber 6 will monotonically increase as the liquid level falls.In other words, as the fuel level changes, the optical impedance betweenside-emitting optical fiber 4 and side-receiving optical fiber 6 willchange. These changes in optical impedance in turn produce changes inthe optical power (i.e., light intensity) output by the side-receivingoptical fiber 6 to the optical detector 14.

Although not depicted in FIG. 1, each optical fiber is a flexible,optically transparent or translucent fiber made of extruded glass orplastic. It can function as a waveguide or light pipe to transmit lightbetween the two ends of the fiber. The term “optical fiber” as usedherein refers to a cylindrical dielectric waveguide that transmits lightalong its axis. The fiber consists of a transparent core surrounded by atransparent cladding layer (hereinafter “cladding”), both of which aremade of dielectric materials. Light is kept in the core by thephenomenon of total internal reflection. To confine the optical signalin the core, the refractive index of the core is greater than that ofthe cladding. The boundary between the core and cladding may either beabrupt, as in step-index fiber, or gradual, as in graded-index fiber.The embodiments disclosed herein employ plastic optical fibers. Plasticoptical fibers have high transmission capacity, excellent immunity toelectromagnetic interference-induced noise, light weight, highmechanical strength, and outstanding flexibility. Plastic optical fibersare also larger in diameter as compared to glass optical fibers. Due totheir larger diameters, plastic optical fibers have greater tolerancefor fiber misalignment than glass optical fibers have. Because of thislarge misalignment tolerance, plastic optical fiber-based networks havelower maintenance and installation costs. In aerospace platforms,plastic optical fibers also greatly reduce the cost of connectors andtransceiver components used in an avionics network. In alternativeembodiments, glass optical fibers can be used in place of plasticoptical fibers.

The systems and methods disclosed herein also utilize side-emittingoptical fibers. The side-emitting optical fibers utilized herein have aplastic or glass core clad with a material that is different than thematerial of the core. To enable side emission, scattering features areintroduced into the optical fiber at various locations. In accordancewith one method, the core region is doped with small refractive and/orreflective light-scattering particles during manufacture. Alternatively,the surface of the core is modified or treated to have surface featuresthat scatter light out of the core. Some examples of light-emittingsurface features include serrations, notches, scratches, texture,roughness, corrugations, etching, abrasion, etc. The entire length offiber can be modified or treated to have side-emitting properties, orjust a portion of the fiber (i.e., a portion along the length orcircumference of the fiber, or both). Side-emitting optical fibers alsoinherently function in reverse, i.e., as “side-receiving” opticalfibers, because the same features that scatter light out of the opticalfiber (i.e., when illuminated from one end) can also scatter light intothe optical fiber (i.e., when illuminated from the side). However,although in theory a side-emitting optical fiber also qualifies as aside-receiving optical fiber, as used herein the term “side-emittingoptical fiber” will be used to refer to an optical fiber that receiveslight at one end and emits at least some of that light from the side,while the term “side-receiving optical fiber” will be used to refer toan optical fiber that receives light from the side and emits at leastsome of that light from one end.

In accordance with the embodiments disclosed herein, the cladding of theside-emitting optical fiber 4 is modified (e.g., by roughening ornotching the circumferential surface) to enable a controlled level ofradial light output along the fiber's length. More specifically, thecladding of side-emitting optical fiber 4 may be treated to produce anon-uniform surface at least in an area bounded by a longitudinal slotin a jacket. For example, the outer surface of the cladding may beroughened or notched at least in an area overlapped by a longitudinalslot in a jacket, thereby forming a side window, as will be described inmore detail below with reference to FIGS. 10 and 11. The cladding of theside-receiving optical fiber 6 may be modified in a similar manner toform a side window that faces toward the side window of theside-emitting optical fiber 4 when the optical sensor is installedinside a fuel tank.

In addition or in the alternative, the side-receiving optical fiber 6can be a fluorescent fiber having a core containing fluorescing dopants,which can be activated by light from the side-emitting optical fiber 4impinging on the side window of the side-receiving optical fiber 6 andthen entering the core of the side-receiving optical fiber 6.(Fluorescence occurs when an orbital electron relaxes to its groundstate by emitting a photon of light after being excited to a higherquantum state by some type of energy.) The fluorescing dopants producelight which travels along the length of the side-receiving optical fiber6 and is then output to the optical detector 14.

At any given axial position along the length of the side-emittingoptical fiber 4, the circumferential variation in the emitted light ispreferably strongly peaked in a narrow angular range subtended by theside window formed by modification of the cladding of the side-emittingoptical fiber 4. As previously mentioned, this side window can be formedby modifying the cladding of the optical fibers (e.g., by notching,scratching or sanding) on only one side to more easily emit light withan angular spread that impinges on a corresponding side window formed bymodification of the cladding of the side-receiving optical fiber 6.

The theoretical underpinning of the optical impedance fuel level sensorconcept will now be described with reference to FIG. 2, which shows aside-emitting optical fiber 4 and a side-receiving optical fiber 6separated by a distance d inside a fuel tank 20 that is partly filledwith fuel 24. A typical diameter of side-emitting optical fiber 4 andside-receiving optical fiber 6 is 1 mm. In the configuration depicted inFIG. 2, light from a light source (not shown), having an input opticalpower P_(in), is input to the side-emitting optical fiber 4. Thehorizontal arrows in FIG. 2 represent the propagation of side-emittedlight from side-emitting optical fiber 4. The light output from theside-receiving optical fiber 6 has an output optical power P_(out) thatis highest when the fuel tank 20 is empty. As the fuel level rises, theoutput optical power P_(out) decreases. By measuring the change inP_(out), the fuel level change can be derived.

In the example shown in FIG. 2, optical fibers are used to measure thelevel of fuel in a fuel tank. In other embodiments, the same apparatusmay be used to detect other liquids. For example, the system describedabove may be used to detect the presence of water in a container orhydraulic fluids in a reservoir for a hydraulic system. The illustrationof detecting fuel in a fuel tank is presented for purposes ofillustration and not meant to limit the manner in which the system shownin FIG. 2 may be used.

In FIG. 2, the following dimensions are indicated: the fuel level is h;and the total length of the side-emitting optical fiber 4 and of theside-receiving optical fiber 6 is set equal to H, since the end faces ofthe two optical fibers are close to the bottom of the fuel tank 20 and His close to the height of the fuel tank 20. The relationship of outputoptical power P_(out) versus fuel level h is a function of theside-emitting efficiency per unit area of the side-emitting opticalfiber 4, the photo response efficiency per unit area of theside-receiving optical fiber 6, and other factors. Physically, as thefuel level changes, P_(out) is the summation of the output optical powerP_(out/air) due to absorption of photons from the side-emitting opticalfiber 4 by the air 22 and the output optical power P_(out/fuel) due toabsorption of photons from the side-emitting optical fiber 4 by the fuel24, i.e., P_(out)=P_(out/air)+P_(out/fuel).

In principle, a single side-emitting optical fiber 4 and a singleside-receiving optical fiber 6 should be able to provide the fuel levelinformation based on the detected output optical power P_(out) of theside-receiving optical fiber 6. But in a real airplane fuel tank, thereare issues of fuel gunk and residue which can build up on the surfacesof the side-emitting optical fiber and side-receiving optical fiber.This build-up obscures the fuel level (h) measuring accuracy. Anotherconsideration is that the quality of fuel used in an airplane in servicecan change over time because different countries may provide differentgrades of fuel at their airports. In addition, the sensor system shouldhave a stable light source (laser or LED) to provide a proper opticalpower input P_(in) to the side-emitting optical fiber 4 for measuringfuel level h. Also, over time the optical fibers can age and theside-emitting optical fiber emitting efficiency and the side-receivingoptical fiber response efficiency can be degraded over time.

To overcome these issues, differential spectral fuel level sensors canbe installed in a fuel tank which operate by causing light having twodifferent wavelengths to propagate along the same optical path in thefuel. More specifically, two different narrow wavelength ranges havingrespective center wavelengths which have different spectral attenuationsin the fuel are selected. By computing a ratio of the received opticalpowers of the respective wavelengths, the intensity variations thataffect the received power reading and therefore the fuel level accuracycan be neutralized.

FIG. 3 is a hybrid diagram representing some components of adifferential spectral liquid level sensor 2 a in accordance with a firstexample embodiment. FIG. 3 depicts some of the factors which may causevariations in the optical intensity of the light propagating from theside-emitting optical fiber 4 into the side-receiving optical fiber 6,such as surface tension wetting 42 (non-shedding of liquid), fuel gunkbuildup 44 on the optical window surface of the fiber sensor elements,and ice slush 46 in the lower portion of the fuel tank due to watercondensation and cold temperature.

The differential spectral liquid level sensor 2 a depicted in FIG. 3comprises at least the following components: a first optical source 10 athat outputs light comprising a first wavelength; a second opticalsource 10 b that outputs light comprising a second wavelength differentthan the first wavelength; a side-emitting optical fiber 4 that isoptically coupled to the first and second optical sources; a firstoptical filter 26 a that passes light having the first wavelength, butdoes not pass light having the second wavelength; a first opticaldetector 28 a that receives light that has passed through the firstoptical filter 26 a; a second optical filter 26 b that passes lighthaving the second wavelength, but does not pass light having the firstwavelength; a second optical detector 28 b that receives light that haspassed through the second optical filter 26 b; and a side-receivingoptical fiber 6 that is optically coupled to the first and secondoptical detectors by way of the first and second optical filtersrespectively. The first and second optical detectors 28 a and 28 bconvert impinging light into first and second electrical signalsrespectively. The first and second optical filters 26 a and 26 b andfirst and second optical detectors 28 a and 28 b form a differentialreceiver that outputs the first and second electrical signals to acomputer system (not shown in FIG. 3). The computer system is configuredto calculate an estimated level of liquid in the reservoir based on adifference of the first and second electrical signals output by thedifferential receiver.

The light output by the first and second optical sources 10 a and 10 bis guided into the side-emitting optical fiber 4 by way of an opticalY-combiner 32. The optical Y-combiner 32 has two input branches (notshown in FIG. 3) which are respectively optically coupled to the firstand second optical sources 10 a and 10 b by respective optical fibers 30a and 30 b. The optical Y-combiner 32 has an output branch (not shown inFIG. 3) which is optically coupled to the side-emitting optical fiber 4by an optical fiber 34. Each of the optical fibers 10 a, 10 b and 34 maycomprise a single length of optical fiber or a plurality of lengths ofoptical fiber connected in series by optical connectors (not shown inthe drawings). A portion of the light that enters the upper end of theside-emitting optical fiber 4 is emitted toward the side-receivingoptical fiber 6. A portion of the light that enters the side-receivingoptical fiber 6 exits the upper end of the side-receiving optical fiber6. The light output by the side-receiving optical fiber 6 is guided intothe first and second optical detectors 28 a and 28 b by way of anoptical Y-splitter 38. The optical Y-splitter 38 has an input branch(not shown in FIG. 3) which is optically coupled to the side-receivingoptical fiber 6 by an optical fiber 36. The optical Y-splitter 38 hastwo output branches which are respectively optically coupled to thefirst and second optical filters 26 a and 26 b by respective opticalfibers 40 a and 40 b. Each of the optical fibers 36, 40 a and 40 b maycomprise a single length of optical fiber or a plurality of lengths ofoptical fiber connected in series by optical connectors (not shown inthe drawings).

In the design shown in FIG. 3, if acrylic plastic optical fiber isselected, visible wavelengths (e.g., red, green, blue etc.) have lessattenuation than infrared and should be chosen. If glass fiber orperfluorinated plastic fiber is selected, invisible infrared wavelengthshave less attenuation and should be chosen. The two wavelengths shouldbe chosen such that they have high contrast of absorption in fuel togive a high optical power ratio for the respective first and secondoptical detectors 28 a and 28 b. Since the light beam comprising thefirst wavelength and the light beam comprising the second wavelengthlight propagate through the same optical path (assuming that opticalfibers 30 a and 30 b are identical and that optical fibers 40 a and 40 bare identical), all common-mode intensity variation effects (describedhereinabove) that create error in the received power can be neutralized.Only the fuel level in the meniscus tube 8 that modulates the opticalsignal between the side-emitting and side-receiving optical fibersresults in changes to the light that impinges respectively on the firstand second optical detectors 28 a and 28 b. The cladding of theside-emitting optical fiber 4 is modified to enable a controlled levelof light output along the fiber's length. The side-receiving opticalfiber 6 can be fabricated the same way or can be a fluorescent opticalfiber that collects light along the length of the fiber and transmitsother light having a different wavelength to the upper end of theside-receiving optical fiber 6. Each bottom end of the side-emitting andside-receiving optical fibers can be fitted with a mirror 18 (see FIG.9) to reduce the optical power loss out that fiber end.

Fluorescence is the emission of light by a substance that has absorbedlight or other electromagnetic radiation. As used herein, the term“fluorescent optical fiber” means an optical fiber that comprises a coresurrounded by cladding, wherein the core is doped with special materialsthat will produce light (i.e., photons) having a first spectralbandwidth centered at a first wavelength when light having a secondspectral bandwidth centered at a second wavelength different than thefirst wavelength is absorbed by that core. Typically the firstwavelength is greater than the second wavelength. In accordance withalternative embodiments, fluorescent glass optical fibers can be usedinstead of fluorescent plastic optical fiber.

FIG. 4 is a diagram representing a top view of an optical Y-coupler 80in accordance with one embodiment. The optical Y-coupler 80 comprisesthree optical fibers 82, 84 and 86. Although not visible in FIG. 4, theend faces of optical fibers 82 and 84 will be bonded and opticallycoupled to one end face of optical fiber 86. The optical Y-coupler 80 isdesigned to facilitate the propagation of incoming light by internalreflection.

In accordance with one proposed implementation, the optical fibers 82,84 and 56 are made of plastic and have a diameter of 1 mm except alongrespective end sections of optical fibers 82 and 84. Each of the opticalfibers 82 and 84 comprise respective end sections where fiber materialhas been removed to form respective planar faces and respectivesemicircular end faces. The end sections begin where the circular crosssections of optical fibers 82 and 84 transition to non-circular andterminate at the respective semicircular end faces. More specifically,the end section of optical fiber 82 is shaped to form a first side facethat intersects and is perpendicular to the semicircular end face ofoptical fiber 82, while the end section of plastic optical fiber 84 isshaped to form a second side face that intersects and is perpendicularto the semicircular end face of optical fiber 84. These side faces arebonded and optically coupled to each other by a first layer of indexmatching epoxy 88. The semicircular end faces of the optical fibers 82and 84 combine to form a circular end face that is bonded and opticallycoupled to a circular end face of the optical fiber 86 by a second layerof index matching epoxy 90, which eliminates back reflection.

The optical Y-combiner 32 and optical Y-splitter 38 (see FIG. 3) mayeach be constructed as shown in FIG. 4. When optical Y-coupler 80 isbeing used as a combiner, it guides any light that enters plastic fibers82 and 84 from the left-hand side (as viewed in FIG. 4) into, throughand out of the plastic fiber 86, thereby combining the light from twooptical sources. Conversely, when optical Y-coupler 80 is being used asa splitter, it guides any light that enters plastic fiber 86 from theright-hand side (as viewed in FIG. 4) into, through and out of theplastic fibers 82 and 84, thereby splitting the light from theside-receiving optical fiber.

In accordance with a second example embodiment, instead of two differentlight sources employing an optical Y-coupler or a dichroic mirror, asingle broadband light source can be used such as a white LED or RGB LEDfor visible wavelengths, or a BLED (broadband LED) for infraredwavelengths. Multiple wavelengths are contained within the broadbandlight source and the two differential wavelengths will be filtered outfor use by two optical detectors with associated wavelength filters.FIG. 5 is a hybrid diagram representing some components of adifferential spectral liquid level sensor 2 b in accordance with thesecond example embodiment comprising at least the following components:a broadband optical source 10 c that outputs light comprising abandwidth that encompasses first and second wavelengths; a side-emittingoptical fiber 4 that is optically coupled to the broadband opticalsource 10 c; a first optical filter 26 a that passes light having thefirst wavelength, but does not pass light having the second wavelength;a first optical detector 28 a that receives light that has passedthrough the first optical filter 26 a; a second optical filter 26 b thatpasses light having the second wavelength, but does not pass lighthaving the first wavelength; a second optical detector 28 b thatreceives light that has passed through the second optical filter 26 b;and a side-receiving optical fiber 6 that is optically coupled to thefirst and second optical detectors by way of the first and secondoptical filters respectively. The light output by the broadband opticalsource 10 c is guided into the side-emitting optical fiber 4 by way ofan optical fiber 34. A portion of the light that enters the upper end ofthe side-emitting optical fiber 4 is emitted toward the side-receivingoptical fiber 6. A portion of the light that enters the side-receivingoptical fiber 6 exits the upper end of the side-receiving optical fiber6. The light output by the side-receiving optical fiber 6 is guided intothe first and second optical detectors 28 a and 28 b by way of anoptical Y-splitter 38. The optical Y-splitter 38 has an input branch(not shown in FIG. 5) which is optically coupled to the side-receivingoptical fiber 6 by an optical fiber 36. The optical Y-splitter 38 hastwo output branches which are respectively optically coupled to thefirst and second optical filters 26 a and 26 b by respective opticalfibers 40 a and 40 b. The first and second optical detectors 28 a and 28b convert impinging light into electrical signals which are sent to acomputer system for processing.

Instead of using an optical Y-splitter 38 and a pair of optical filters26 a and 26 b to split the optical signals exiting the upper end of theside-receiving optical fiber 6 as shown in FIGS. 3 and 5, other types ofoptical devices can be used. For example, FIG. 8 shows the equivalentfunctionality of a dichroic mirror 48. The dichroic mirror 48 has acoating that allows light having a first wavelength (e.g., blue light)to go through and reflects light having a second wavelength differentthan the first wavelength (e.g., green light). The light of firstwavelength would pass through the dichroic mirror 48 and onto the firstoptical detector 28 a, while the light of second wavelength would bereflected by the dichroic mirror 48 onto the second optical detector 28b. The dichroic mirror 48 and the first and second optical filters 26 aand 26 b form a differential receiver.

In accordance with a third example embodiment, instead of two differentwavelength filters and two optical detectors, a single optical detectoris used for receiving both wavelengths from two light sources that aretime-division multiplexed onto the same optical fiber. On the receiveside, the time-division multiplexed optical signals are demultiplexed bymeans of synchronized switches to allow the respective optical powersignals due to the respective wavelengths to be extracted and separatelyprocessed by a computer system. The ratio of those separate opticalpowers can then be calculated, which ratio is indicative of the fuellevel in a properly calibrated system.

FIG. 6 is a hybrid diagram representing some components of adifferential spectral liquid level sensor 2 c in accordance with a thirdexample embodiment comprising at least the following components: a firstoptical source 10 a that outputs light comprising a first wavelength; asecond optical source 10 b that outputs light comprising a secondwavelength different than the first wavelength; a time-divisionmultiplexing (TDM) controller 80 configured to control the first andsecond optical sources 10 a and 10 b to output time-division multiplexedoptical pulses having the first and second wavelengths in alternatingsequence; a side-emitting optical fiber 4 that is optically coupled tothe first and second optical sources 10 a and 10 b; a side-receivingoptical fiber 6 that is positioned parallel to and at a distance fromthe side-emitting optical fiber 4; an optical detector 28 that isoptically coupled to receive light from the side-receiving optical fiber6; and a time-division demultiplexer 82 that is electrically coupled tothe optical detector 28 and comprises switches which are controlled todemultiplex the time-division-multiplexed electrical signals output fromthe optical detector 28. The fuel level measurement system furthercomprises a computer system 62 configured to calculate an estimatedlevel of fuel in the fuel tank based on a difference of thedemultiplexed signals output from the time-division demultiplexer 82.

Each optical source may comprise an LED and an LED driver that receivescontrol signals from the TDM controller 80. The respectivetime-division-multiplexed optical pulses output by the first and secondoptical sources 10 a and 10 b are guided into the side-emitting opticalfiber 4 by way of an optical Y-combiner 32. The optical Y-combiner 32has two input branches (not shown in FIG. 3) which are respectivelyoptically coupled to the first and second optical sources 10 a and 10 bby respective optical fibers 30 a and 30 b. The optical Y-combiner 32has an output branch (not shown in FIG. 3) which is optically coupled tothe side-emitting optical fiber 4 by an optical fiber 34. A portion ofthe light that enters the upper end of the side-emitting optical fiber 4is emitted toward the side-receiving optical fiber 6. A portion of thelight that enters the side-receiving optical fiber 6 exits the upper endof the side-receiving optical fiber 6. The light output by theside-receiving optical fiber 6 is guided into the optical detector 28 byway of an optical fiber 36. The optical detector 28 converts impingingoptical pulses into electrical signals which are sent to thetime-division demultiplexer 82.

FIG. 7 is a graph showing two optical pulses in accordance with atime-division multiplexing scheme used in the third example embodimentpartly depicted in FIG. 6. The vertical axis measures optical pulseamplitude; the horizontal axis measures time. The optical pulse widthst₁ and t₂, as well as the period of each pulse train and time interval Tbetween the two different color optical pulses, can be chosen tooptimize the response for each wavelength and for the power ratio of thetwo optical signals. To improve linearization response in fuel, theoptical sources can be driven with a current signal waveform having anexponential falling edge, corresponding to the exponential attenuationof optical power as the emitted light propagates through the fuelcolumn.

As shown in FIG. 9, a reflective mirror cap 18 may be attached to thebottom end of the side-emitting optical fiber 4 to reflect light backthrough side-emitting optical fiber 4 and to prevent light from beinglost out the bottom end. A similar reflective cap may be attached to thebottom end of the side-receiving optical fiber 6 to reflect light backthrough the side-receiving optical fiber 6 toward the optical detector14 (see FIG. 3).

FIG. 10 is a diagram representing a plan view of a pair of straightlight guides of an optical sensor in accordance with an embodimentintended for use in the measurement of a level of a liquid that will notdamage exposed optical fibers when the latter are immersed in theliquid. The transmitting light guide comprises: a side-emitting opticalfiber 4 having an axis and a circumferential surface; and a jacket 66having a longitudinal slot 68 that extends parallel to the axis of theside-emitting optical fiber 4 for the entire length of the latter.Preferably the longitudinal slot 68 overlaps a side window formed by anon-uniform surface on the cladding of the side-emitting optical fiber4. The jacket 66 is in contact with and covers the circumferentialsurface of the side-emitting optical fiber 4 except in the area oflongitudinal slot 68. The transmitting light guide may further comprisea curved reflective surface disposed between the side-emitting opticalfiber 4 and the jacket 66. Preferably the jacket 66 is made of amaterial which is not optically transparent or translucent, such asmetal or polymeric material.

Similarly, the receiving waveguide comprises: a side-receiving opticalfiber 6 having an axis and a circumferential surface; and a jacket 70having a longitudinal slot 72 that extends parallel to the axis of theside-receiving optical fiber 6 for the entire length of the latter.Preferably the longitudinal slot 72 overlaps the side window formed by anon-uniform surface on the cladding of the side-receiving optical fiber6. The jacket 70 is in contact with the circumferential surface of theside-receiving optical fiber 6 except in an area of the longitudinalslot 72. The receiving waveguide may further comprise a curvedreflective surface disposed between the side-receiving optical fiber 6and the jacket 70. Preferably the jacket 70 is made of a material whichis not optically transparent or translucent, such as metal or polymericmaterial.

In the case where the jackets 66 and 70 are made of polymeric material,those jackets can be formed by molding. The side-emitting andside-receiving optical fibers may each have a circular, square orhexagonal cross section, with the molded jacket conforming to the shapeof the optical fiber.

The arrows in FIG. 10 represent light which has been emitted byside-emitting optical fiber 4 through the side window formed in thecladding of the side-emitting optical fiber 4 and is propagating throughintervening fluid (e.g., liquid or air) toward the corresponding sidewindow formed in the cladding of side-receiving optical fiber 6.However, it should be appreciated that, in the absence of a focusinglens overlying the side window of the side-emitting optical fiber 4, theexiting rays of light may be divergent, rather than collimated.

FIG. 11 is a diagram representing a plan view of a pair of straightlight guides of an optical sensor in accordance with an embodiment inwhich the liquid is not in direct contact with the side-emitting andside-receiving optical fibers 4 and 6. The only difference from theembodiment depicted in FIG. 10 is that the transmitting and receivinglight guides further comprise respective lenses 74 and 76 formed (e.g.,by molding) in the longitudinal slots of the respective jackets 66 and70. Preferably the lenses 74 and 76 extend the full length of thelongitudinal slots. In combination, lens 74 and jacket 66 encase theside-emitting optical fiber 4, with lens 74 interfacing with the sidewindow of side-emitting optical fiber 4. Similarly, lens 76 and jacket70 encase the side-receiving optical fiber 6, with lens 76 interfacingwith the side window of side-receiving optical fiber 6. Preferably thelenses 74 and 76 are made of epoxy.

The arrows in FIG. 11 represent light which has been emitted byside-emitting optical fiber 4 through the lens 74 and is propagatingthrough intervening fluid (e.g., liquid or air) toward the lens 76 ofthe receiving light guide. The lens 74 may be designed so that exitingrays of light are directed in parallel toward the lens 76. The lens 76may be designed so that impinging parallel rays of light are convergedinto the side-receiving optical fiber 6. The lenses have the effect ofincreasing the intensity of the light output by side-receiving opticalfiber 6 for the same optical power being pumped into side-emittingoptical fiber 4, thereby enhancing the performance of the opticalimpedance sensor.

If the optical power transmitted by a high-intensity LED is adequate,then the system may comprise a single side-emitting optical fiberdisposed parallel with one side-receiving optical fiber. If the opticalpower from one LED is inadequate, then the amount of light emitted canbe increased in various ways. In some embodiments, the system maycomprise two or more side-emitting optical fibers surrounding acentrally located side-receiving optical fiber. In this case theside-receiving optical fiber is collecting light from all sides, andeach side-emitting optical fiber has its own set of differential opticalsources. In these alternative embodiments, the signal-to-noise ratio ofthe optical impedance sensor is increased by employing multipleside-emitting and/or side-receiving optical fibers.

Any one of the above-described fuel level sensors may be installed in afuel tank onboard an airplane along with a temperature sensor and adensitometer. The fuel level and fuel density data and geometry of thefuel tank can then be used to compute the estimated quantity (i.e.,mass) of fuel in the fuel tank. (In order to measure the mass of thefuel for engine consumption and range calculation, the system can usemeasurements of fuel level and fuel density.) In addition, an airplanecan be retrofit by removing existing electrical fuel level sensors andinstalling optical fuel level sensors in their place. In accordance withone fuel level sensor configuration, the locations of the respectivesensors in the wing tank and the center tank of an airplane dictate thesensor height and therefore fiber sensor length. In the baselineconfiguration there would be a one-to-one replacement of each electricalsensor by an optical sensor. The double-shielded electrical wiring forthe electrical sensor will be replaced by lightweight optical fiber,eliminating weight from the wiring and supporting brackets, andeliminating electromagnetic effects from lightning, shorting, fraying ofelectrical wiring. The use of optical fibers instead of electrical wiresalso eliminates any safety hazards due to electrical fault conditions.Although glass fiber can be used, plastic optical fiber is more flexibleand more forgiving for installation in the very tight space of anairplane fuel tank.

FIG. 12 is a block diagram representing components of a system formeasuring a quantity of fuel in a fuel tank in accordance with oneembodiment. The system comprises a fuel level sensor 52 (of a typedescribed above) that outputs electrical signals representing the levelof fuel in a fuel tank, a densitometer 54 that outputs electricalsignals representing the density of the fuel in the fuel tank and atemperature sensor 56 that outputs electrical signals representing thetemperature of the fuel in the fuel tank. Each of these sensors may beincorporated in a respective line replaceable unit (LRU). These LRUs areconnected to a remote data concentrator (RDC) 58.

In accordance with one implementation, the RDC 58 is connected to acomputer system 62 (e.g., a fuel quantity processing unit) by way of amulti-master serial bus known as a CAN bus 60. For this purpose, the RDC58 and the computer system 62 may each incorporate a controller and atransceiver of the type used in a controller area network (CAN). Such aCAN controller and CAN transceiver are referred to herein as a “CANnode”. The RDC 58 has different dedicated analog circuits to separatelymeasure the temperature, density, and level of the fuel. The analogvalues of these parameters are converted to digital values, packed in adata field and transmitted via the CAN bus 60 to the computer system 62.Note that ARINC 845 CAN bus is just an example of a simple avionicdigital data bus that can be used and that any other digital data bussuch as ARINC 425 or ARINC 664 can be used as well.

In accordance with the CAN communications protocol, each CAN node isable to send and receive messages, but not simultaneously. A message orframe consists primarily of an identifier, which represents the priorityof the message, and a number of data bytes. The message is transmittedserially onto the CAN bus 60 by the CAN transceiver and may be receivedby all CAN nodes. Each CAN node connected to CAN bus 60 waits for aprescribed period of inactivity before attempting to send a message. Ifthere is a collision (i.e., if two nodes try to send messages at thesame time), the collision is resolved through a bit-wise arbitration,based on a preprogrammed priority of each message in the identifierfield of the message. The message that contains the highest priorityidentifier always wins bus access.

The sensor data acquired by fuel level sensor 52, densitometer 54 andtemperature sensor 56 is formatted in accordance with the CANcommunications protocol to form CAN messages, which are broadcast ontothe CAN bus 60 and received by the computer system 62. The computersystem 62 is configured to estimate the mass of fuel remaining in thefuel tank (or compartment thereof) based on the measured fuel density,the known geometry of the fuel tank (or compartment thereof) and themeasured fuel level h. For example, the volume of fuel remaining can becomputed based on the known geometry and measured fuel level, and thenthe mass of fuel remaining will be equal to the product of volume anddensity. An electrical signal representing the estimated mass ofremaining fuel is output from the computer system 62 to a fuel gauge 64.The fuel gauge 64 may take the form of a display device having a displayprocessor programmed to display the measurement results (e.g., the fuellevel or the fuel quantity) graphically and/or alphanumerically on adisplay screen.

In accordance with one proposed implementation, a differential spectralfuel level sensor is installed in a compartment of a fuel tank. Eachoptical source is in the form of a transmit integrated circuit connectedto a transmit optical subassembly (comprising a laser or LED). Eachoptical detector is in the form of a receive integrated circuitconnected to a receive optical subassembly (comprising a photodiode).The magnitude of the fuel level signals output by the differentialspectral fuel level sensor increases monotonically with increasingintensity of light emitted from the end of the side-receiving opticalfiber 6. The computer system 62 may be a dedicated microprocessor or ageneral-purpose computer configured to perform differential processingof signals representing the respective optical powers for the respectivefirst and second wavelengths. This differential processing removes theundesirable effects of any common-mode intensity variations. The resultsof the differential processing are then used to calculate the measuredlevel (i.e., height) of the fuel by using a look-up table, a calibrationcurve, or by solving equations, as appropriate.

The computer system 62 may be a computer or part of a flight controlsystem located on an aircraft. In identifying the amount of fuel presentin an irregular-shaped fuel tank, the computer system 62 may executevarious routines to calculate the amount of fuel present based onoptical power data received from multiple side-receiving optical fibersappropriately placed in various compartments of the fuel tank. The fuelinformation processing software may include routines that take intoaccount the shape of the fuel tank to determine the amount of fuelremaining in the fuel tank. The fuel information processing software mayfurther include routines for calibrating processes to form a baselinebefore a first use or to maintain accuracy of fuel readings. Thereadings provided by the computer system 62 to the fuel gauge 64 may beintegrated or averaged before presentation and may be provided atdifferent time intervals.

While optical fuel level sensors have been described with reference tovarious embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the teachings herein. Inaddition, many modifications may be made to adapt the concepts andreductions to practice disclosed herein to a particular situation.Accordingly, it is intended that the subject matter covered by theclaims not be limited to the disclosed embodiments.

The embodiments disclosed above use one or more computing systems. Asused in the claims, the term “computing system” comprises one or more ofthe following: a computer, a processor, a controller, a centralprocessing unit, a microcontroller, a reduced instruction set computerprocessor, an ASIC, a programmable logic circuit, an FPGA, a digitalsignal processor, and/or any other circuit or processing device capableof executing the functions described herein. For example, a computingsystem may comprise multiple microcontrollers or multiple processorswhich communicate via a network or bus. As used herein, the terms“computer” and “processor” both refer to devices having a processingunit (e.g., a central processing unit) and some form of memory (i.e.,computer-readable medium) for storing a program which is readable by theprocessing unit.

The methods described herein may be encoded as executable instructionsembodied in a non-transitory tangible computer-readable storage medium,including, without limitation, a storage device and/or a memory device.Such instructions, when executed by a processing or computing system,cause the system device to perform at least a portion of the methodsdescribed herein.

The process claims set forth hereinafter should not be construed torequire that the steps recited therein be performed in alphabeticalorder (any alphabetical ordering in the claims is used solely for thepurpose of referencing previously recited steps) or in the order inwhich they are recited unless the claim language explicitly specifies orstates conditions indicating a particular order in which some or all ofthose steps are performed. Nor should the process claims be construed toexclude any portions of two or more steps being performed concurrentlyor alternatingly unless the claim language explicitly states a conditionthat precludes such an interpretation.

The invention claimed is:
 1. A system for measuring a level of liquid ina reservoir, comprising: a first optical source that outputs lightcomprising a first wavelength; a second optical source that outputslight comprising a second wavelength different than the firstwavelength; a time-division multiplexing controller configured tocontrol the first and second optical sources to output time-divisionmultiplexed optical pulses having the first and second wavelengths inalternating sequence; a side-emitting optical fiber that is opticallycoupled to the first and second optical sources; a side-receivingoptical fiber that is positioned parallel to and at a distance from theside-emitting optical fiber; an optical detector that is opticallycoupled to receive light from the side-receiving optical fiber andconvert the received light to electrical signals; and a time-divisiondemultiplexer that is electrically coupled to the optical detector andcomprises switches which are controlled to demultiplex the electricalsignals output by the optical detector, wherein the optical detectorconverts unfiltered received light emitted from one end of theside-receiving optical fiber to the electrical signals in response tooutput of an optical pulse having the first wavelength and in responseto output of an optical pulse having the second wavelength inalternating sequence.
 2. The system as recited in claim 1, furthercomprising a computer system configured to calculate an estimated levelof liquid in the reservoir based on a difference of demultiplexedelectrical signals output from the time-division demultiplexer.
 3. Thesystem as recited in claim 2, further comprising a display deviceelectrically coupled to the computing system, wherein the computingsystem is further configured to execute the following operations:storing data representing a geometry of the reservoir; receiving datarepresenting a measurement of a density of the liquid in the reservoir;calculating a mass of liquid remaining in the reservoir based on thegeometry of the reservoir, the density of the liquid and the estimatedlevel of liquid; and outputting an electrical signal representing thecalculated mass of liquid in the reservoir to the display device.
 4. Thesystem as recited in claim 3, wherein the liquid is fuel and thereservoir is a fuel tank onboard an airplane.
 5. The system as recitedin claim 1, wherein the light comprising the first wavelength and thelight comprising the second wavelength have different attenuations whenpropagating through the liquid.
 6. The system as recited in claim 1,further comprising an optical Y-combiner having first and second inputbranches optically coupled to the first and second optical sourcesrespectively and an output branch optically coupled to the side-emittingoptical fiber.
 7. The system as recited in claim 1, further comprising ameniscus tube surrounding the side-emitting optical fiber and theside-receiving optical fiber.
 8. A method for measuring a height ofliquid in a reservoir, comprising: placing a side-emitting optical fiberand a side-receiving optical fiber in the reservoir having respectivelocations whereat the side-emitting optical fiber and side-receivingoptical fiber are mutually parallel and separated by a distance; guidinga series of time-division-multiplexed optical pulses into one end of theside-emitting optical fiber, wherein the time-division-multiplexedoptical pulses comprise alternating optical pulses having a firstwavelength and a second wavelength respectively; side-emitting at leastsome of the optical pulses received by the side-emitting optical fibertoward the side-receiving optical fiber; guiding unfilteredtime-division-multiplexed optical pulses received and output by theside-receiving optical fiber onto an optical detector; convertingtime-division-multiplexed optical pulses that impinge on the opticaldetector into time-division-multiplexed electrical signals;demultiplexing the time-division-multiplexed electrical signals outputby the optical detector to generate first and second electrical signals;and calculating an estimated level of liquid in the reservoir based on adifference of the first and second electrical signals.
 9. The method asrecited in claim 8, further comprising: storing data representing ageometry of the reservoir; measuring a density of the liquid in thereservoir; calculating a mass of liquid remaining in the reservoir basedon the geometry of the reservoir, the density of the liquid and theestimated level of liquid; and displaying a gauge that indicates thecalculated mass of liquid in the reservoir.
 10. The method as recited inclaim 9, wherein the liquid is fuel and the reservoir is a fuel tankonboard an airplane.
 11. The system as recited in claim 1, wherein theliquid is fuel and the reservoir is a fuel tank onboard an airplane. 12.The system as recited in claim 11, further comprising a meniscus tubesurrounding the side-emitting optical fiber and the side-receivingoptical fiber.
 13. The system as recited in claim 12, wherein themeniscus tube extends to a floor of the fuel tank and has openings whichallow fuel to flow into a volume of space bounded by the meniscus tube.14. The system as recited in claim 2, wherein the liquid is fuel and thereservoir is a fuel tank onboard an airplane.
 15. The system as recitedin claim 14, further comprising a meniscus tube surrounding theside-emitting optical fiber and the side-receiving optical fiber. 16.The system as recited in claim 15, wherein the meniscus tube extends toa floor of the fuel tank and has openings which allow fuel to flow intoa volume of space bounded by the meniscus tube.
 17. The system asrecited in claim 1, wherein the side-emitting optical fiber is made ofplastic and comprises a core that is doped with light-scatteringparticles.
 18. The system as recited in claim 17, wherein theside-receiving optical fiber is made of plastic and comprises a corethat contains fluorescing dopants.
 19. The system as recited in claim 1,wherein the side-emitting optical fiber is made of plastic and comprisesa core having a surface that has been modified or treated to havesurface features that scatter light out of the core.
 20. The system asrecited in claim 19, wherein the side-receiving optical fiber is made ofplastic and comprises a core that contains fluorescing dopants.